Tire Size Calculator
Going from 16" wheels to 18"? Enter your stock tire size and instantly see the closest equivalent sizes for every wheel diameter — plus exactly how far off your speedometer will read with each one.
Your stock tire size Free
Stock tire specs
Already have a size in mind?
Skip the suggestions below and compare your stock size directly against any specific size you're considering — useful if a tire shop quoted you something not shown in the common-sizes list.
Note: this checks the math for any size, even ones tire makers don't actually produce. The suggestions below only show sizes commonly stocked by major retailers.
Equivalent sizes by wheel diameter
Only shows sizes actually stocked by major tire retailers — not every size that's mathematically close. Tap any size for the speedometer error table and a visual comparison. Rule of thumb: stay within ±3% of stock diameter (within ±1% is ideal).
Even within the sizes above, exact stock and pricing vary a lot by brand and by season — a size that's cheap and in stock at one retailer might be back-ordered at another. It pays to check two or three sellers before buying.
Comparison
| Speedometer reads | Actual speed |
|---|
Ready to buy? Availability and price for the same exact size can swing by $30–80 a tire between sellers. Compare a couple options before you check out.
Compare on CarGurus →Quick answers
How do I read a tire size like 225/60R16?
225 is the tread width in millimeters. 60 is the aspect ratio — the sidewall height as a percentage of the width (here, 60% of 225mm = 135mm tall). R16 means it fits a 16-inch wheel. Overall diameter = wheel + two sidewalls.
Why does tire size affect my speedometer?
Your speedometer counts wheel rotations and assumes the stock tire circumference. A larger tire covers more ground per rotation, so your speedometer reads lower than your true speed. A smaller tire does the opposite. Even 2–3% matters: at a true 70 mph that's about 2 mph of error — enough to earn a ticket or fail a state inspection in some places.
What is "plus sizing"?
Going up one inch in wheel diameter while keeping the overall tire diameter nearly the same. The pattern: +1 inch of wheel ≈ +10mm width and −5 to −10 aspect ratio. Example: 225/60R16 → 225/55R17 → 245/45R18 → 245/40R19. Bigger wheel, shorter sidewall, same rollout.
Will a wider tire fit my car?
This calculator matches diameters, but width clearance depends on your specific wheel offset, suspension, and fender room. As a rule, +10 to +20mm over stock is usually fine; beyond that, check a fitment forum for your exact model or measure clearance at full steering lock.
Why don't I see every mathematically close size?
Tire makers don't produce every possible width/aspect-ratio combination — only specific sizes get manufactured and stocked at scale. This tool matches your stock diameter against sizes that are actually common in retailer catalogs, so what you see here is realistically shoppable. Even so, exact in-stock availability and pricing shift often, so it's worth checking a couple of sellers before you commit to a size.